Temperature compensated sense amplifier

ABSTRACT

A temperature compensated, phase tolerant sense amplifier for use in a magnetic bubble memory system in which current is applied to the detector resistors only during a bubble detect operation.

This is a division of application Ser. No. 90,958, filed Nov. 5, 1979,now U.S. Pat. No. 4,261,044.

Related subject matter is contained in U.S. patent application Ser. No.90,959, filed Nov. 11, 1979, entitled "A Phase Tolerant Magnetic BubbleMemory Sense Amplifier", now U.S. Pat. No. 4,318,187.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a magnetic bubble memorysense amplifier and, more particularly, to a temperature compensatedmagnetic bubble memory sense amplifier.

2. Prior Art Statement

In a typical form, magnetic bubble memory systems include a magneticbubble memory device in which magnetic bubbles are stored and retrievedunder the control of an appropriate control circuit. During the bubbleretrieval operation, the control circuit moves the magnetic bubblesstored in the magnetic bubble memory device in a conventional manner sothat a previously-stored sequence of bubbles and voids are passedbeneath a detector resistor formed on the surface of the device. Inresponse to the movement of a magnetic bubble therebeneath, the detectorresistor briefly changes in resistance value, as compared to a dummyresistor which is also formed on the magnetic bubble memory device tohave substantially the same nominal resistance as the detector resistor.This induced differential voltage change is detected by a senseamplifier constructed to provide an output signal when the differentialexceeds a predetermined threshold voltage. Typically, however, the senseamplifier must identify the desired differential voltage change in thepresence of a substantial common-mode signal produced by the accessfield and other high-frequency noise, as well as an almost unavoidableoffset voltage.

In the past, attempts were made to utilize sense amplifiers which weredesigned for other applications, such as the Motorola MC1444 Plated WireSense Amplifier, to perform the magnetic bubble detection function.Dissatisfaction led to the development of special sense amplifiers foruse in magnetic bubble memory systems, such as the Texas InstrumentsSN75281 Bubble-Memory Sense Amplifier. However, such sense amplifiershave been less than fully satisfactory due to excessive offset voltagesensitivity, undesirable input impedance loading characteristics, orother similar deficiencies. More recently developed sense amplifiers,such as those used in the Intel 7242 Formatter/Sense Amplifier, promisesome improvement, but have yet to be fully evaluated.

In addition, an overview of magnetic bubble memory systems and theiroperations may be found in the following U.S. Pat. Nos. 4,152,776;4,156,935; and 4,159,537.

SUMMARY OF THE INVENTION

In a magnetic bubble memory system in which magnetic bubbles are movedbeneath a detector resistor to change the resistance thereof relative toa dummy resistor under the control of a control circuit, an improvedsense amplifier includes an amplifier portion which provides an outputsignal proportionally greater than the voltages developed across thedetector and dummy resistors via respective current sources, acomparator portion for providing a trigger signal indicative of adetected magnetic bubble in response to an applied comparator inputsignal which exceeds a predetermined threshold level, and a temperaturecompensating portion which adjusts the current provided by the currentsources in relation to a sensed temperature. In one alternateembodiment, the temperature compensating portion may, eitheralternatively or additionally, adjust the threshold level of thecomparator portion in relation to the sensed temperature. In one otherembodiment, the temperature compensation portion may, again eitheralternatively or additionally, adjust the gain of the amplifier portionin relation to the sensed temperature. In any of the embodiments, thesensed temperature may be either the temperature of the temperaturesensing portion itself or the temperature of the magnetic bubble memorydevice.

It is an object of the present invention to provide an improved senseamplifier for use in the magnetic bubble memory system.

Another object of the present invention is to provide an magnetic bubblememory sense amplifier having temperature compensation.

Yet another object of the present invention is to provide a magneticbubble memory sense amplifier which provides a flexible means forcompensating for temperature-induced drift in the output signalsprovided by the bubble detectors of a magnetic bubble memory device.

Still another object of the present invention is to provide a magneticbubble memory sense amplifier having an externally adjustable form oftemperature compensation.

Other objects and advantages of the present invention will be evidentfrom the following detailed description when read in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block representation of a magnetic bubble memory systemincluding an improved sense amplifier constructed in accordance with thepreferred embodiment of the present invention.

FIG. 2 is a set of wave form diagrams depicting the time-relatedoperation of the sense amplifier of FIG. 1.

FIG. 3 is a preferred form of the peak detecting portion of the senseamplifier of FIG. 1.

FIG. 4 is a preferred form of the temperature compensation and currentcontrol portions of the sense amplifier of FIG. 1.

FIG. 5 is a preferred form of a temperature compensated comparator foruse in the sense amplifier of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Shown in a simplified schematic form of FIG. 1 is a magnetic bubblememory system comprising a magnetic bubble memory device 10, a controlcircuit 12, and an improved sense amplifier 14 which is constructed inaccordance with the preferred embodiment of the present invention. As istypical in such magnetic bubble memory devices as the Texas InstrumentsTIB0203 92 K-Bit Magnetic Bubble Memory, the magnetic bubble memorydevice 10 operates under the control of the control circuit 12 togenerate and store a sequence of magnetic bubbles and voids, during awrite operation. During a subsequent read operation, the magnetic bubblememory device 10 operates under the control of the control circuit 12 toretrieve a stored sequence of magnetic bubbles and voids, and to movethe retrieved sequence beneath a detector resistor 16. Typically, thedetector resistor 16 is formed on the surface of the magnetic bubblememory device 10 using a conventional magneto-resistive material whichchanges resistance in response to the movement of a magnetic bubbletherebeneath. Thus, the movement of a magnetic bubble beneath thedetector resistor 16 causes a brief fluxuation in the voltage developedthereacross via an electrical current flowing therethrough. Ordinarily,the bubble-induced peak-to-peak voltage fluxuation is typically of theorder of about 3 mV in the normal loaded operating condition. In aneffort to ease the task of detecting the bubble-induced voltagefluxuation, a dummy resistor 18 having substantially the same nominalresistance as the detector resistor 16 is provided on the magneticbubble memory device 10, and oriented so that the noise-inducedfluxuations in the voltages developed across the detector resistor 16and the dummy resistor 18 tend to cancel. To realize this benefit, thedetector resistor 16 and the dummy resistor 18 are generally connectedin a bridge network, with the detector resistor 16 and the dummyresistor 18 being connected in series with current sources 20 and 22which provide current flow through the respective resistors to developthe desired voltages thereacross.

An amplifier 24 is capacitively coupled via input capacitors 26 and 28to the detector resistor 16 and the dummy resistor 18 so as to beresponsive to fluxuations of the voltages developed thereacross. Theamplifier 24 provides an output signal which is proportionally greaterthan the difference in the voltages developed across the resistors 16and 18. In the preferred form, the amplifier 24 is differential in formso that the output signal provided thereby is differential rather thansingle-ended. In addition, it is preferred that the gain of theamplifier 24 be established by a temperature compensation circuit 26 ina manner to be described hereinafter.

A comparator 28 is capacitively coupled via coupling capacitors 30 and32 to the output of the amplifier 24, so that the amplifier outputsignal may be applied to the comparator 28 as a comparator input signal.The comparator 28 provides a trigger signal for application to a latch34 via a set gate 36 in response to the comparator input signalexceeding a predetermined threshold level. In the preferred form, thecomparator 28 is constructed so that the threshold voltage may beestablished by the temperature compensation circuit 26 as set forthhereinafter.

Referring in FIG. 2 to the detector wave form as provided by theamplifier 24, it can be seen that the voltage fluxuation developedacross the detector resistor 16, relative to the dummy resistor 18, bythe movement of a magnetic bubble therebeneath tends to deviate from anominal voltage toward a first relative voltage extreme or peak 38, andthen towards a second opposite relative voltage extreme or peak 40,before again returning to the nominal voltage. In an effort toaccurately determine the actual peak-to-peak voltage deviation betweenthe peak 38 and the peak 40, various attempts have been made to isolatethe comparator 28 from the amplifier 24 until the occurance of the peak38. In one form, a peak clamp 42 is provided for clamping the comparatorinput signal to a predetermined peak clamp voltage in response to a peakclamp signal. In such prior art devices, the peak clamp signal has beenprovided by the control circuit 12 at a fixed time relative to thecontrolled movement of the magnetic bubbles. It has been discovered,however, that manufacturing variations present in actual magnetic bubblememory devices 10 tend to induce shifts in the phase of the detectorwave form, so that the clamping signal provided by the control circuit12 typically occurs before or after the actual peak 38. In the preferredform of the present invention, however, a peak detector 44 detects therelative voltage extreme of the amplifier output signal associated withthe peak 38, and provides the peak clamp signal for the peak clamp 42when the voltage of the amplifier output signal is at least equal to thedetected relative voltage extreme.

Assuming that the peak detector 44 is constructed to be responsive tothe relatively negative peak 38, it can be seen in FIG. 2 that thevoltage of the amplifier output signal increases in absolute valuerelative to the nominal voltage level until the relative voltage extremeoccurs at the peak 38. Thereafter, the voltage of the amplifier outputsignal is less than the detected relative voltage extreme, causing thepeak detector 44 to disable the peak clamp 42. Of course, the peakdetector 44 could be constructed to respond in a similar manner to apositive peak if the detector wave form were inverted relative to thenominal voltage level.

In the preferred form, the control circuit 12 does provide a clampsignal (C) of predetermined duration prior to each movement of themagnetic bubbles stored in the magnetic bubble memory device 10. In thisform, the peak detector 44 applies the peak clamp signal to the peakclamp 42 in response to the clamp signal, as well as when the amplifieroutput signal has a voltage at least equal to the detected relativevoltage extreme. With reference to FIG. 2, the clamp signal ispreferably initiated at a time T₁ which is shortly before the actualbubble movement, and terminated at a time T₃ which is shortly before theearliest anticipated occurance of the peak 38 at a time T₄. In thismanner, the peak detector 44 is forced to actuate the peak clamp 42 froma time prior to actual bubble movement to a time shortly before the peak38, and thereafter actuates the peak clamp 42 only until the actualoccurance of the peak 38 at T₄. The resulting combination prevents thepeak detector 44 from deactivating the peak clamp 42 as a result ofdetecting any noise-induced relative voltage extremes which may occurbetween the initiation of the clamp signal at T₁ and the terminationthereof at T₃.

In addition to the peak clamp 42, the preferred form of the senseamplifier 14 includes an input clamp 46 for clamping the voltagescapacitively coupled to the amplifier 24 to an input clamp voltage inthe absence of a strobe signal (S). In this form, the control circuit 12provides the strobe signal from a selected time during the period of theclamp signal until a selected time following the clamp signal, theduration of the strobe signal effectively defining the time intervalduring which the sense amplifier 14 will be responsive to the detectoroutput signal. With reference to FIG. 2, the strobe signal is preferablyinitiated at a time T₂ shortly after the initiation of the clamp signaland terminated at a time T₆ following the anticipated return of thedetect or output signal to the nominal voltage level. In this manner,the input clamp 46 is enabled except for the time period during whichthe bubble-induced voltage fluxuation is present in the detector outputsignal.

The brief period of coincidence of the clamp and strobe signals, betweenT₂ and T₃, provides a convenient opportunity to reset the latch 34 via areset gate 48 which is responsive to the simultaneous presence of boththe clamp and strobe signals. Thereafter, the presence of the strobesignal without the clamp signal enables the set gate 36 so that atrigger signal provided by the comparator 28 may set the latch 34. Withreference to FIG. 2, the reset gate 48 will reset the latch 34 at thetime T₂, and the set gate 36 will enable the setting of the latch 34 atthe time T₃. If the comparator input signal thereafter exceeds thethreshold voltage V_(T), as shown for example at the time T₅, the latch34 will be set and remain set until the next coincidence of the clampand strobe signals.

In the preferred form, each of the current sources 20 and 22 provides aflow of current through the respective resistor 16 and 18, generally inresponse to control signals provided by the control circuit 12. Moreparticularly, the current sources 20 and 22 provide current at a leveldetermined by a reference voltage provided by the temperaturecompensation circuit 22, generally under the control of a gate 50 whichis responsive to the clamp and strobe signals applied thereto via a NORgate 52. In this form, the gate 50 will couple the reference voltage tothe current sources 20 and 22 when either the clamp signal or the strobesignal is being provided by the control circuit 12. Otherwise, the gate50 will apply a disable voltage to the current sources 20 and 22 whichis selected to disable the current sources 20 and 22, so that no currentflows through either the detector resistor 16 or the dummy resistor 18.Thus, current flow through the detector resistor 16 and dummy resistor18 will be initiated at the time T₁ and terminated at the time T₆, withno current being provided thereafter until the next occurance of theclamp signal. In this pulsed current mode of operation, resistiveheating of the detector resistor 16 and dummy resistor 18 is minimized,thereby reducing the operating margin loss and the temperature-induceddrift of the output voltage levels. Preferably, the reference voltageprovided by the temperature compensation circuit 26 is selected using anexternal reference resistor 54, which may be fixed, variable, ortemperature responsive, as desired.

Shown in FIG. 3 is a preferred form for the peak detecting portion ofthe sense amplifier shown in FIG. 1. More particularly, the peakdetector 44 is comprised of a buffer 56 which couples the detectoroutput portion of the differential amplifier output signal to one sideof a differentiating capacitor 58. An inverter 60, responsive to theclamp signal (C), has an open collector output coupled to the other sideof the differentiating capacitor 58. A second inverter 62 has a clampedinput portion coupled to the output of the inverter 60, and an outputportion which provides drive current for a pair of clamp transistors 64and 66 in the peak clamp 42 which are interposed between a groundreference and the sides of the coupling capacitors 30 and 32 connectedto the comparator 28. If appropriate, one or more biasing diodes 68 and70 may be interposed in series with the transistors 64 and 66 toestablish the peak clamp voltage at a desired level above the groundreference. In operation, the inverter 60 will maintain the input to theinverter 62 at a low level as long as the clamp signal is applied to theinput thereof. However, the buffer 56 and capacitor 58 will continue tohold the input to the inverter 62 low as long as the detector outputportion of the differential amplifier output signal continues to movenegatively relative to the nominal voltage level, that is, until thetime T₄ shown in FIG. 2. Thereafter, the relatively positive movement ofthe detector output portion of the differential amplifier output signalcauses the buffer 5 to pull the input to the inverter 62 high via thedifferentiating capacitor 58. Of course, the inverter 62 will supplydrive current to the transistors 64 and 66 only when the input theretois held low either by the inverter 60 or by the buffer 56 anddifferentiating capacitor 58.

Shown in FIG. 4 is a preferred form of the temperature compensationcircuit 26, the current sources 20 and 22, and the gate 50. In thetemperature compensation circuit 26, a voltage reference 72 ofconventional form provides a very precise reference voltage relative tothe ground reference. Although this reference voltage may be applieddirectly to the current sources 20 and 22 via a current amplifyingbuffer 74, it has been found preferable to apply the reference voltageto the base of a temperature-responsive biasing transistor 76 interposedin series with an internal biasing resistor 78 and the external biasingresistor 54, generally between a positive potential and the groundreference. Since the base to emitter voltage, V_(BE), of the transistor76 varies in a well known manner with temperature, the voltage appliedto the buffer 74 via the collector of the transistor 76 varies with thetemperature of the temperature compensation circuit 26. Thus, if thetemperature compensation circuit 26, and particularly the transistor 76,is disposed relatively close to the magnetic bubble memory device 10,then the reference voltage applied to the current sources 20 and 22varies in a manner which tends to increase the flow of current throughthe resistors 16 and 18 as the temperature thereof increases. Thisincreased flow of current tends to counteract the natural reduction inthe bubble-induced voltage fluxuations across the resistors 16 atincreasing temperatures. A particularly convenient manner fortemperature compensating the detector output signal is to provide theresistor 54 in the form of a thermistor which is thermally coupled tothe package containing the magnetic bubble memory device 10.

The reference voltage provided by the buffer 74 of the temperaturecompensation circuit 26 is applied to a pair of transistors 80 and 82which are connected in series with resistors 84 and 86 to formrespective constant current sources 20 and 22. The gate 50 may consistsimply of a shunt transistor 88 interposed between the output of thebuffer 74 and the ground reference, so that the current provided by thebuffer 74 is shunted away from the transistors 80 and 82 when the NORgate 52 provides drive current in response to the simultaneous absenceof both the clamp and the strobe signals.

Shown in FIG. 5 is one preferred form for a temperature compensatedcomparator 28a which may be used as the comparator 28 in the senseamplifier 14 of FIG. 1. More particularly, the comparator 28a has a pairof input transistors 90 and 92, the bases of which are coupled to thecoupling capacitors 30 and 32, respectively. The emitter of the inputtransistor 90 is coupled via a biasing resistor 94 to the base of adifferential transistor 96 and the emitter of the input transistor 92 isdirectly coupled to the base of a differential transistor 98, while thecollectors of each of the input transistors 90 and 92 is connecteddirectly to a positive potential. The collectors of each of thedifferential transistors 96 and 98 are coupled via respective pull-upresistors 100 and 102 to the positive potential, while the emitters ofthe differential transistors 96 and 98 are coupled to the groundreference via a constant current source 104. A pair of biasingtransistors 106 and 108 are connected in series with biasing resistors110 and 112, respectively, between the ground reference and the bases ofthe differential transistors 96 and 98. If the reference voltageprovided by the voltage reference 72 of the temperature compensationcircuit 26 shown in FIG. 4 is applied to the bases of the transistors106 and 108, a fixed level of biasing current at a precise bias voltageis drawn from the bases of the differential transistors 96 and 98. As aresult, a fixed offset voltage is established by the bias resistor 94 atthe base of the differential transistor 96 relative to the base of thedifferential transistor 98. This offset voltage effectively defines thethreshold voltage by which the voltage applied to the base of the inputtransistor 90 must exceed the voltage applied to the base of the inputtransistor 92 before the conducting state of the differential transistor96 changes relative to the conducting state of the differentialtransistor 98. Thus, the voltage at the collector of the differentialtransistor 98 may be employed as the trigger signal to indicate when thedetector output portion of the differential amplifier output signal,coupled via the coupling capacitor 30 to the input transistor 90,exceeds the voltage of the dummy output portion, coupled via thecoupling capacitor 32 to the input transistor 92, by the thresholdvoltage established by the bias resistor 94. On the other hand, if thetemperature compensated reference voltage available at the collector ofthe transistor 76 in the temperature compensation circuit 26 of FIG. 4is applied to the biasing transistors 106 and 108, the resultantvariation in the biasing currents and voltages applied to the bases ofthe differential transistors 96 and 98 effectively varies the thresholdvoltage established by the bias resistor 94. Thus, the threshold voltagecan be made to vary inversely with temperature to compensate for thereduction in the bubble-induced peak-to-peak voltage fluxuations withincreasing temperature.

Although the improved sense amplifier 14 has been disclosed in detailherein in combination with a particular form of magnetic bubble memorysystem, it will be clear to those skilled in the art that the presentinvention may be advantageously employed in other types of systems. Itis intended therefore, that variations or changes that may be made inthe arrangement or construction of the parts or elements of the improvedsense amplifier 14, to adapt the circuit to other uses be consideredwithin the scope of the present invention as defined in the appendedclaims.

What is claimed is:
 1. A sense amplifier for providing a trigger signaloutput, said sense amplifier comprising:current source means forproviding a flow of current at a level determined by a reference voltagethrough each of first and second conductors to develop respectivevoltages therefor; amplifier means having first and second inputsconnected to said current source means via said first and secondconductors for providing an intermediate output signal proportionallygreater than the differential signal developed between said first andsecond inputs from the respective voltages of said first and secondconductors; comparator means connected to the output of said amplifiermeans for providing the trigger signal output in response to theintermediate output signal from said amplifier means exceeding apredetermined threshold voltage; and temperature compensation meansconnected to said current source means for providing the referencevoltage thereto at a voltage magnitude determined by the temperaturesensed by said temperature compensation means.
 2. A sense amplifier asset forth in claim 1, wherein said temperature compensation means isconnected to said amplifier means for establishing the gain of saidamplifier means at a level determined by the temperature sensed by saidtemperature compensation means.
 3. A sense amplifier as set forth inclaim 1, wherein said temperature compensation means is connected tosaid comparator means for providing the threshold voltage for saidcomparator means at a voltage magnitude determined by the temperaturesensed by said temperature compensation means.
 4. A sense amplifier asset forth in claim 3, wherein said temperature compensation means isalso connected to said amplifier means for establishing the gain of saidamplifier means at a level determined by the temperature sensed by saidtemperature compensation means.
 5. A sense amplifier for providing atrigger signal output, said sense amplifier comprising:current sourcemeans for providing a flow of current at a level determined by areference voltage through each of first and second conductors to developrespective voltages therefor; amplifier means having first and secondinputs connected to said current source means via said first and secondconductors for providing an intermediate output signal proportionallygreater than the differential signal developed between said first andsecond inputs from the respective voltages of said first and secondconductors; comparator means connected to the output of said amplifiermeans for providing the trigger signal output in response to theintermediate output signal from said amplifier means exceeding apredetermined threshold voltage; and temperature compensation meansconnected to said amplifier means for establishing the gain of saidamplifier means at a level determined by the temperature sensed by saidtemperature compensation means.
 6. A sense amplifier as set forth inclaim 5, wherein said temperature compensation means is connected tosaid comparator means for providing the threshold voltage for saidcomparator means at a voltage magnitude determined by the temperaturesensed by said temperature compensation means.
 7. A sense amplifier forproviding a trigger signal output, said sense amplifiercomprising:current source means for providing a flow of current at alevel determined by a reference voltage through each of first and secondconductors to develop respective voltages therefor; amplifier meanshaving first and second inputs connected to said current source meansvia said first and second conductors for providing an intermediateoutput signal proportionally greater than the differential signaldeveloped between said first and second inputs from the respectivevoltages of said first and second conductors; comparator means connectedto the output of said amplifier means for providing the trigger signaloutput in response to the intermediate output signal from said amplifiermeans exceeding a predetermined threshold voltage; and temperaturecompensation means connected to said comparator means for providing thethreshold voltage for said comparator means at a voltage magnitudedetermined by the temperature sensed by said temperature compensationmeans.
 8. A sense amplifier as set forth in any of claims 1, 5, and 7,wherein the temperature sensed by said temperature compensation means isthe temperature of said temperature compensation means itself.